(1) Field of the Invention
The present invention relates to an improved method of characterizing defects in semiconductors or insulators.
(2) Description of the Prior Art
Thermally stimulated current (TSC) is a well known method for the characterization of defects in semiconductors or insulators. A sample is excited either optically or electrically to fill up the defect states or traps with carriers at a low temperature, e.g. the temperature of liquid nitrogen. Then the sample is heated with the temperature increasing linearly with time. A current due to carriers being emitted from the filled defects or traps is then recorded as a function of temperature.
Usually, a bias voltage is needed as the driving force for emitting carriers during TSC measurement. However, a parasitic current which originates from the bias voltage and increases exponentially with temperature can cover up the TSC signal, especially for energetically deep defects in the band gap. Zero-bias thermally stimulated current (ZBTSC) makes use of either a built-in potential or a temperature gradient as the driving force for carriers emitted during measurement. It has been found that a parasitic current still exists for ZBTSC due to a very small temperature gradient accidentally created in the sample during ZBTSC measurement. During ZBTSC measurement, a probe is needed to contact the device under test (DUT) which is placed on the cold finger of a cryostat. A cryostat is a piece of equipment used for lowering the temperature to, for example, the temperature of liquid nitrogen. There is a temperature difference between the probe holder and the sample holder, the so-called cold finger. The temperature of the cold finger can be lowered to, for example, the temperature of liquid nitrogen. This temperature difference can produce a small temperature gradient across the DUT, resulting in a parasitic current in the ZBTSC spectrum.
FIG. 1 graphically illustrates the TSC signal at increasing temperature during ZBTSC measurement. The TSC peaks 11 and 12 indicate defect states. 13 indicates the parasitic current due to the temperature gradient in the sample during temperature scan. The parasitic current 13 will mask the TSC peak 12.
The parasitic current during ZBTSC limits the range of ZBTSC to relatively energetically shallow defects. For example, the defects at low temperatures (11) may be found while defects at higher temperatures (12) will be masked by the parasitic current 13. Sometimes, the parasitic current can be the detection limit for even energetically shallow defects. It is desired to find a way to provide zero-temperature-gradient ZBTSC.
A number of patents discuss the TSC technique. U.S. Pat. No. 4,839,588 to Jantsch et al states that there is always a certain amount of leakage current present when TSC is reverse biased. This patent teaches a method of measuring change in a microwave field rather than the current measurement of TSC. U.S. Pat. No. 5,360,267 to Ibar discusses the use of TSC to measure relaxation of especially plastics and polymers. U.S. Pat. No. 4,496,642 to Tam et al shows a TSC method for measuring current as temperature is increased. No mention is made of leakage current or temperature gradient.